The present invention relates generally to current mode hysteresis controllers used, for example, to control alternating current (AC) motors that are powered through static inverters by direct current (DC) power sources such as batteries or rectified AC power and, more particularly, to current band control within such controllers. While the present invention is generally applicable to controlling inverters for example to drive single phase motors and two phase motors, it will be described herein with reference to the control of a three phase motor for which it is being initially applied.
AC motors in comparison to DC motors are generally of simpler structure and include advantages of lower cost, more compact size, less weight and operation at higher speeds and efficiencies. However, controllers for AC motors have been complex and expensive which has hampered the adoption of AC motors for use with DC supplies such as batteries and in particular for applications in motor vehicles. Simplification of AC motor controllers has resulted from the use of current mode controllers.
For current mode control, the current flowing into motor terminals is measured and compared to reference current signals which are generated by a motor controller and are representative of desired current for the motor to effect desired operating conditions. Error signals resulting from the comparison are used to switch semiconductor elements of an inverter circuit which provides power to the motor. Pulse width modulated (PWM) DC power is thus rapidly switched to the terminals of the motor.
One popular form of current mode control uses hysteresis comparators. In hysteresis current mode control, the actual currents within a motor are maintained within a band of given amplitude centered on reference currents which are representative of desired current levels within the motor. An example of hysteresis current mode control for use in a motor vehicle and in particular to control an electrically powered active suspension for a vehicle is disclosed in U.S. Pat. No. 5,027,048 which issued to Masrur et al on Jun. 25, 1991, is assigned to Ford Motor Company and is incorporated herein by reference. The hysteresis motor controller of the Masrur et al patent also discloses use of field oriented control which is another important improvement in the control of AC motors.
A problem encountered in hysteresis current mode control is the selection of the hysteresis band. The bandwidth of the hysteresis band is selected to provide adequate control of the current level and also to minimize inverter losses. For steady-state operation at current levels greater than the hysteresis band, selection of the bandwidth is no problem. However, if current levels vary, a compromise selection must be made. Even for a compromise bandwidth, in applications such as an active suspension, reference current levels can frequently go below the hysteresis band such that the inverter is coasting, modulation disappears and road disturbances are not compensated by the suspension system.
Accordingly, there is a need for an improved current mode hysteresis controller to substantially reduce inverter coasting while still providing high efficiency tightly controlled system operation.